Supporting characters are genuinely effective supporters in anime such as Kaguya-Sama: Love Is War. A good supplementary character isn't just there to fill in a scene or give the protagonist a sounding board. Dynamic and active characters such as Chika Fujiwara, Miko Iino and Yu Ishigami make the entire premise richer and more memorable.
Of course, Kaguya-Sama's main focus is on the humorous but stubborn battle of wits between Kaguya Shinomiya and Miyuki Shirogane, making this rom-com the Death Note of a generation. But without Chika Fujiwara on the stage, the co-protagonists' love war would get stale and overblown quickly.
Contrasts are vital in Kaguya-Sama, and the two sides of its conflict are order and chaos. It's a universal and well-known dichotomy, and neither one works without the other. On one hand, Kaguya and Miyuki represent order, using their crafty schemes, mind games, pre-planning and ambitious ideas to outwit the other and move the story (and their budding relationship) along.
But if this was the entire show, then Kaguya-Sama would soon run out of ideas, or at least become stale. Cunning mind games are always exciting, but eventually, Kaguya and Miyuki's battle of wits would become tedious and take itself too seriously. Likewise, if everything goes according to plan for one party or the other, then the tension is all but lost. This situation can only have so much humor in it. Enter Chika Fujiwara and the rest of the supporting cast.
Chika serves the broad role of adding chaos and whimsical elements to the show, helping keep the order in check and throwing off everyone's expectations. This completes the Kaguya-Sama narrative, defying viewers' anticipations and adding more humor and new ideas to a given scene or plotline. It's a well-rounded package, and while every supporting character plays this role, no one does it better than Chika. Hence why she's introduced right away, even before the likes of Yu Ishigami and the disciplinarian Miko Iino.
For the sake of humor and suspense, the narrative of Kaguya-Sama constantly challenges and disrupts the schemes of both Kaguya and Miyuki -- primarily through Chika (and sometimes Yu, Miko or even Ai Hayasaka). Kaguya and Miyuki are two of a kind, and while their direct battles are entertaining enough, Chika's third-party nature makes her the ultimate challenge. She's not taking sides in the story's central love battle, and in fact, she's not even aware of it at all. Chika cannot be bought easily, nor can she be tamed or predicted. This ensures that Kaguya and Miyuki each have two opponents to juggle, not just one.
Chika's chaos challenges Miyuki and Kaguya in ways they cannot challenge each other, and while they don't find this very funny, viewers certainly do. Chika's whimsical tastes and love of games add energy and suspense to every scene, from memory games to love quizzes to brainstorming new ideas for group vacations. No one, not even Chika herself, can guess how all this will turn out. In this way she draws out the best in both Kaguya and Miyuki, unwittingly daring them to handle each other and Chika's chaos while keeping a straight face and adjusting to unexpected developments.
This is a good mental exercise, and it helps Kaguya and Miyuki learn not to take themselves too seriously (though they are learning this lesson slowly). Chika keeps them honest with her chaotic natural energy and colorful whims. How do you tame the untamable? Kaguya and Miyuki can't win in the face of Chika's chaos, but they will sure try, and they and Kaguya-Sama as a whole are better off for it.